1. Field of the Invention
The present invention relates to a method of measuring mechanical stresses in measuring objects of hardened steel using magnetic transducers of the type which comprises a magnetizing circuit for generating a magnetic field in the measuring object and a measuring circuit for sensing the changes in the magnetic field which occur in the object being measured when the object is subjected to mechanical forces.
2. The Prior Art
A transducer of this type is shown and described in U.S. Pat. No. 2,912,642.
Normally a transducer of the type shown and described in U.S. Pat. No. 2,912,642 is used for measuring the torque on the propeller shafts on board ships. Such shafts normally have a carbon content of about 0.4% and are in most cases normalized. The internal structure is then composed of ferrite and pearlite, which are both ferromagnetic. As a rule a well-defined connection between stresses in the shaft surface, that is, the torque, and the output signal of the transducer is obtained for such steel. In exceptional cases a so-called creep may be observed. By creep is meant that the signal is to some extent time-dependent so that, after an instantaneous change in the torque, a certain time -- of the order of magnitude of a few seconds up to about 1 hour -- elapses before the signal has become stable. For soft steel the creep, when it occurs, is positive, that is, the delayed part of the signal change has the same direction as the immediate change. The rate of the creep, that is, the relation between the delayed and the immediate signal change, in this case seldom exceeds a few tenths per cent.
If, on the other hand, the transducer of the above patent is used for measuring shear stresses occurring because of torsion or bending in hardened steel shafts, a negative creep of the signal will occur and this creep is of the order of magnitude of 1% at full magnetization. At low magnetization the creep will disappear, but the neutral point is instead very sensitive to any momentary increase in the magnetization, either caused by an intentional momentary increase of the current, or in the form of a transient current increase caused by a momentary interruption in the exciting current.